This invention relates to apparatus for manufacturing laminated composite structures in general, and in particular to methods and apparatus for manufacturing composite structures utilizing layers of composite tape. Still more particularly, this invention relates to apparatus for forming concave tape wrapped composite structures.
Laminated composite structures are currently employed in a variety of applications in which their lightweight, high strength, and other physical characteristics are of benefit. In recent years there has been an increasing interest in the use of composite structures in the place of metal components of aircraft, ground vehicles, and structural members in which a high strength-to-weight ratio is of importance. Because of fuel conservation considerations, for example, the utilization of composite structures as components in aircraft is increasing, and major portions of automobile bodies and aircraft components have been constructed of composites. Typically, such composite structures comprise a plastic matrix, of a material such as epoxy, reinforced with a fibrous cloth or tape of a material such as boron, graphite or Fiberglass. The present invention relates to the manufacture of such composite structures by tape laying procedures, i.e., the manufacture of such structures by the placement on a mold of successive layers of resin-impregnated tapes.
In many applications, structural members are required to sustain greater loads at particular locations and along particular axes than at other locations or axes. The tape lay-up process is used advantageously to form such structures because the tapes can be oriented to provide maximum strength at the locations and along the axes of greatest stress, thus reducing the weight of the structure by permitting the use of less material in areas not subject to such concentrated loads. The tapes have substantially unidirectional filaments impregnated with uncured or partially cured resin and are generally adhered to a mold as parallel, mutually adjacent lengths or strips. The lengths of tapes of successive layers advantageously extend in differing directions. Successive layers are laid over the mold until a desired thickness is attained, after which the work piece is cured to form a composite structure having reinforcing fibers extending in appropriate directions for sustaining the loads anticipated for the particular structure.
While the advantages of such tape laying methods are known, several limitations and difficulties have in the past tended to limit their general use. One such limitation involves the manufacture of round or circular structures such as an aircraft fuselage member. These structures present difficulties to known composite tape lay-up tachniques due to the mold techniques currently utilized. As those skilled in the art will appreciate, a simple cylindrical mold of constant circumstance can be easily wrapped to form a composite structure. Composite tape can be applied longitudinally, radially and at a constant angle from the normal (typically a positive or negative forty-five degrees). The uncured composite tape can then be cut from the mold, cured and rejoined to form a circular or cylindrical composite structure. A major disadvantage to this technique is the fact that the outer surface of a structure formed with tape in the aforementioned manner is quite rough and not well suited to supersonic aircraft applications. In order to solve this problem it is necessary to utilize molds having concave surfaces and to lay composite tape into the mold. In this manner, the rough last layers of composite tape are on the interior of the finished composite product and the smooth, mold side becomes the exterior when the two halves of a circular or cylindrical structure are joined together.
While the utilization of this concave mold surface technique solves the problem of aerodynamic smoothness for composite structures, it raises new problems with the necessity of accurately laying composite tape inside a concave mold. Several highly complex robotic tape application devices have been proposed for this procedure; however, the complex nature of these systems results in highly costly and complex systems which are difficult to align and which require complicated computerized control systems. Thus, it should be apparent that existing methods and apparatus for forming concave tape wrapped composite structures are highly complex and expensive or are unsatisfactory from an aerodynamic standpoint. Examples of complex tape application devices can be seen in U.S. Pat. Nos. 4,133,711 and 3,775,219.